I'm having problems engaging gears, been told that I probably need a new gear box does anyone know what costs are involved? is it a big job?. fluid levels have been topped up but I have noticed a small leak

You mean clutch not gearbox...

if you have trouble engaging all gears it it will probably not be a gearbox problem, but more likely a clutch problem as thecrow said.

if you have a leak you may be very lucky and simply have a faulty slave cylinder. this is the piston which is on top of the gearbox, it works the clutch when you press the pedal. when the slave cylinder need replacing it is difficult or impossible to engage any gear and brake fluid will leak from it. a slave cylinder is easy to fit and costs approx £35 plus a couple of quid extra for brake fluid and a brake bleed kit if you plan to fit it yourself.

if you are less lucky you may need a new clutch, this requires removal of the gearbox. a clutch kit will cost approx £55 and up to £100 for fitting. not god, however i doubt your clutch will need replacing because a leak would usually indicate a faulty slave cylinder.

you should check to see if the leak is causing the brake fluid level to drop in the reservoir, if it is then the slave cylinder is definately to blame. its fairly common on bravo/as. my cars' slave cylinder failed at only 56000 miles.